Checking For Duplicate In Listbox Item Turns Out Weird Error

I'm trying to add a string to a listbox if there is none exact string existed inside, but it turns out a very weird error. Look at this.
On form_load, I have this:
AddHandler Server1.OnUserLoggedIn, AddressOf HandleUser

And then,
Public Sub HandleUser(ByVal username As String)
'Here, you handle adding the text to a text box.
Me.Invoke(New PrintMessageDelegate(AddressOf AddUser), username)
End Sub
[Code] .....

The output is when I connect the first client, it works fine. But, when I try to connect the second client, it prints out three times both the listbox1 item and the txtDisplay. It's like get caught in loop, but I have tried other kind of loops like for each and other string comparison too, still work that way.